Reinforced iron base alloys containing boron fibers



United States Patent 9 3,476,529 REINFORCED IRON BASE ALLOYS CONTAINING BORON FIBERS Dennis S. Dubin, Merion, and Solomon Frost, Morton,

Pa., assignors to US. Composites Corporation, Philadelphia, Pa., a corporation of Pennsylvania N Drawing. Filed Jan. 11, 1967, Ser. No. 608,501 Int. Cl. B221? 7/02; C22c 39/50 US. Cl. 29183.5 7 Claims ABSTRACT OF THE DISCLOSURE Fibers containing a tungsten or titanium core, an intermediate coating of boron, and an outside coating of titanium or aluminum oxide, reinforced iron base alloys containing such fibers, and a method for making such fibers and such reinforced iron base alloys.

This invention relates to fibers comprising boron, reinforced iron base alloys containing such fibers, and to method for preparing the same, and more particularly to fibers having a tungsten or titanium core, an intermediate coating of boron, and an outside coating of titanium or aluminum oxide, and to reinforced iron base alloys containing such fibers, and to methods of making the same, for the purpose of increasing structural strength with a corresponding reduction of weight. The resulting effect is an increase in the structural strength-weight ratio.

When fibers of uniform structural strength are disposed in a matrix of considerably lower structural strength, the resultant composite will have a final structural in proportion to the percentage volumes of the fibers and matrix. Boron steels are well known and have enhanced structural properties and good machining properties. When the percentage of boron dissolved in steel exceeds approximately 0.01%, the steel becomes brittle and unusable for many purposes. 7 Available boron fibers, prepared by depositing boron onto tungsten wire cores, have a structural strength-weight ratio approximately five to nine times that of steel. Such available boron fibers cannot be inserted into .steel by virtue of the rapidsolubility of boron into the iron matrix to form an interstitial steel alloy.

This invention has as an object the provision of fibers comprising boron and iron base alloy compositions in which very high structural strength to weight ratios may be secured.

This invention has as another object the provision of novel fibers comprising boron which may be inserted into iron base alloys.

Other objects will appear hereinafter.

We have discovered that fibers comprising boron may be incorporated into iron base alloys, such as steel, provided that such fibers are formed of a plurality of concentric material. Such fibers may have a total diameter of between about 0.003 to 0.01 inch, preferably about 0.007 inch. The length dimension of the fibers is not critical, but normally should exceed 20 times the diameter dimension. The fibers may be continuous in length. It is preferred to dispose the fibers in a single direction for unidirectional strength. The subject invention also contemplates theuse of fabrics woven from such fibers to achieve particular characteristics.

The iron base alloys of the present invention comprise the commercial iron alloy products including by way of example and not be way of limitation cast iron, wrought 'ice iron, ingot iron, and steel. Generally, the compositions of the subject invention comprise a matrix which contains 25 to volume percent of iron in the form of an iron base alloy, and from 15 to 75 volume percent of the fibers comprising boron. We have found that a concentration of fibers comprising boron below about 15 volume percent will not give a significant increase in structural strength, and a concentration of fibers comprising boron of more than about 75 volume percent will result in a continuous brittle phase and subsequent unusable material.

The fibers of the present invention comprise a metallic core selected from tungsten or titanium, an intermediate coating of boron, and an outside coating of titanium or aluminum oxide.

The fibers of the present invention may be incorporated into the iron base alloy by conventional means, such as by adding the fibers to molten iron base alloy.

By tungsten as used herein is meant any of the tungsten wires which are now commercially available, and which consist of tungsten or a tungsten alloy having a tungsten content of the order of at least 98 weight percent.

By titanium as used herein is meant titanium or a titanium alloy having a weight percent content of titanium of at least 98 weight percent.

The metallic core of tungsten or titanium is first heated to a temperature of between 1000 and 1300 C., preferably of the order of about 1200 C., under a vacuum or other non-reacting atmosphere, and then reacted with a boron halide and hydrogen to form the intermediate boron coating, followed by the formation of the outside coating.

The core wire of tungsten or titanium should have a diameter of the order of 0.0004 to 0.001 inch. Preferably, it is heated in the reaction chamber by the passage of electrical current through it. A suitable vacuum chamber for the reaction between the tungsten or titanium wire and boron halide and hydrogen comprises a vacuum chamber made up of fused silica glass. Where gasketing materials are needed, suitable gaskets may be formed of Teflon.

A boron halide, such as boron trifiuoride, boron trichloride, or boron tribromide, in the vapor state is mixed with elemental hydrogen and introduced into the chamber containing the heated metallic core. The weight concentration of boron halide in the mixture introduced into the reacting chamber should be of the order of 60 to Weight percent. The concentration of the hydrogen in the gaseous mixture should be of the order of 10 to 40 weight percent in order to prevent hydrogen embrittlement of the core, which may be a problem particularly in the case of a titanium core.

The hydrogen and the boron halide react in the gaseous phase on the hot metal wire and deposit elemental boron onto the hot metal wire, with the hydrogen halide acid being released in the gaseous phase.

The mixture of hydrogen and boron halide is to be passed over the core wire at a rate of 0.1 to 1.0 liter per minute and at a pressure of the order of 10 to 100 millimeters mercury.

It is believed that initially the boron forms an interface of a boride with the metal wire. In order to avoid the complete degradation of the metal wire into its boride, no portion of the metal wire should be at the elevated temperature for more than 30 seconds. Preferably, the time of reaction should be of the order of 5 to 10 seconds.

Following the reaction, the metal composite should be heated for a period of 5 to minutes in an inert atmosphere of pure hydrogen. The temperature is not critical for this last-mentioned heating.

The reaction time should extend sufficiently long so that the resultant boron deposit on the metal Wire is of the order of .001 to .007 inch total thickness taken on the diameter, preferably of the order of .005 inch total thickness taken on the diameter.

We have determined that the concentric tungsten or titanium core and boron wire cannot be used with an iron base alloy because of the rapid dissolution of the boron into the iron matrix to form an interstitial alloy. However, we have discovered that if the aforesaid metal core and boron wires are coated with an outer concentric layer selected from the group consisting of titanium, or aluminum oxide, the fibers may be used for the purpose of providing high structural strength to weight ratios with iron base alloys.

The selection of a coating will depend upon the iron base alloy.

For iron base alloys having a carbon concentration of 0.25 weight percent based on the iron base alloy or less, the outside coating should comprise titanium, which will be bonded to the boron by a thin interface of titanium boride, unless the iron base alloy contains one or more elements of small atomic radii, namely below 1.2 angstrom units, such as beryllium and silicon, in a total excess of .05 weight percent. Such elements having small atomic radii will diffuse through the outer titanium layer, and will form compounds with the underlying boron.

For iron base alloys having a weight percent concentration of carbon of greater than 0.25 weight percent based on the iron base alloy, or when elements having an atomic radii of below 1.2 angstrom units are present to the extent of more than .05 weight percent in the iron base alloy, the outside coating should comprise aluminum oxide.

The titanium (inclusive of the titanium boride interface) should be present as an outer concentric layer of about .0005 to .002 inch total thickness taken on the diameter and preferably about .001 inch total thickness taken on the diameter. The titanium should be coated upon the boron by exposing the filament consisting of a core of tungsten or titanium and a surrounding layer of boron to a temperature of between 1100 and 1350 0, preferably of the order of 1200 C., while in a vacuum, such as a vacuum of the order of 10- to 10 mm. of mercury, and depositing titanium on said surface.

The deposition of titanium on the surface may be accomplished by inserting into the vacuum chamber a relatively thick titanium wire, such as one of the order of 0.1 inch diameter. This relatively thick titanium wire is electrically heated to approximately 1500 C. to 1700 C. or other temperature sufficiently high to effect exaporation of the titanium under the employed vacuum. Titanium then evaporates onto the boron fiber deposit. This deposition is to continue until the requisite thickness of titanium is deposited. The boron fiber is electrically heated for approximately five minutes so that a titanium boride interface is formed between the outer titanium layer and the boron layer.

The outer titanium layer is stable in an inert atmosphere, and will neither decompose nor go into solution in an iron base alloy when the fibers are dispersed in the iron base alloy, provided that such dispersion is accomplished at a temperature of less than 1500 C. and at a time interval of less than about one minute. The iron from the iron base alloy will wet and surface bind with the titanium outer layer when the so-coated fibers are dispersed in the iron base alloy.

A satisfactory aluminum oxide outer coating may be formed from aluminum oxide powders having a mesh size of the order of 200 to 250, with less than 5 weight percent of aluminum silicate binders. The aluminum oxide should be relatively pure. However, mechanical adhesion is somewhat enhanced if trace amounts of soft metals, such as copper, silver or aluminum are included within the aluminum oxide up to a concentration of the order of 1 part by weight in 10,000. The final aluminum oxide coating should be of the order of .0005 to .002 total inch taken on the diameter, and preferably of the order of .001 inch total thickness taken on the diameter.

An example of the manufacture of a suitable aluminum oxide coating on a filament comprises:

Filaments having a tungsten or titanium core surrounded by boron are passed through an aqueous slurry containing about 35 weight percent of aluminum oxide powders having a mesh size of 200 to 250, and containing less than 5 weight percent of aluminum silicate binder. The filaments are coated with this slurry to the extent of an approximate coating of .0005 to .002 total inch taken on the diameter. The fiber is then dried at approximately 150 C. for 10 minutes, or on the order of 100 C. to 200 C. for a comparable time, and then heated to 1200" C. for 10 minutes in an inert atmosphere, or on the order of 800 C. to 1200 C. for a comparable time. This will yield a fiber having a high blossy covering of aluminum oxide.

Aluminum oxide coated filaments of the present invention are to be disposed within molten iron as rapidly as possible, with such disposition to be accomplished in less than 30 seconds.

Other materials can be present in the reinforced iron base alloys of the present invention, such as any of the wide variety of materials added to iron base alloys to accomplish different purposes, such as other metals, oxides, etc.

The iron base alloy compositions of the present invention containing the filaments of the present invention possess very high structural strength to weight ratios. They have excellent utility in applications where high structural strength is necessary, and where weight is a very limiting factor, such as space vehicles.

The present invention may be embodied in other specific forms without departing from the spirit or essential attributes thereof, and, accordingly, reference should be made to the appended claims, rather than to the foregoing specification as indicating the scope of the invention.

It is claimed:

1. A fiber useful for reinforcing an iron base alloy having a diameter of between 0.003 to 0.01 inch, a core having a diameter of between about 0.0004 to 0.001 inch selected from the group consisting of tungsten, alloys of tungsten containing at least 98 weight percent tungsten, titanium, and alloys of titanium containing at least 98 weight percent titanium, an intermediate coating enveloping said core of boron having a total thickness taken on the diameter of between about 0.001 to 0.007 inch, and an outermost coating enveloping said intermediate coating of a total thickness taken on the diameter of between about .0005 to .002 inch, with said outermost coating being selected from the group consisting of aluminum oxide and titanium.

2. A fiber in accordance with claim 1 in which the outermost coating is titanium and is secured to the boron by an interface of titanium boride.

3. A fiber in accordance with claim 1 in which the outermost coating is aluminum oxide containing trace amounts of a metal selected from the group consisting of copper, silver, and aluminum in a concentration of up to 1 part by weight in 10,000.

4. A reinforced iron base alloy comprising from about 25 to about 85 volume percent of an iron base alloy and from about 15 to about volume percent of fibers dispersed therein, each of said fibers having a diameter of between about 0.0004 to 0.001 inch selected from the group consisting of tungsten, alloys of tungsten containing at least 98 weight percent tungsten, titanium, and alloys of titanium containing at least 98 weight percent titanium, an intermediate coating enveloping said core of boron having a total thickness taken on the diameter of between about 0.001 to 0.007 inch, and an outermost coating enveloping said intermediate coating of a total thickness taken on the diameter of between about .0005 to .002 inch, with said outermost coating being selected from the group consisting of aluminum oxide and titanium, with said outermost coating being aluminum oxide when the iron base alloy has more than 0.25 weight percent carbon or contains more than a total of 0.05 weight percent of one or more elements having atomic radii of below 1.2 angstrom units, and with said outermost coating being titanium when said iron base alloy has 0.25 Weight percent carbon or less.

5. A reinforced iron base alloy in accordance with claim 4 in which the fibers are unidirectionally disposed within the iron base alloy.

6. A reinforced iron base alloy in accordance with claim 4 in which the fibers are disposed as a woven fabric.

References Cited UNITED STATES PATENTS 2,455,804 12/1948 Ransley et a] 29191.2 3,098,723 7/1963 Micks 29183.5 3,410,715 11/1968 Hough 117-71 L. DEWAYNE RUTLEDGE, Primary Examiner E. L. WEISE, Assistant Examiner U.S. Cl. X.R. 

